I'm going to ask what may be an obvious question, but wouldn't it make
sense for a serialization of a sequence to correspond on the output to the
serialization on the input? That is to say, if you had a
structure:

("foo",5,<bar><bat/></bar>,<!-- foo
-->)

The
main disadvantage of such a format is that it uses non-XML markup (parentheses
and commas) which makes it difficult to parse using tools that are specialized
to handling XML markup, for example XSLT and XQuery.

Also, it doesn't solve the problem of retaining type annotations,
for example the difference between the integer 5 and the positiveInteger
5.